The present invention relates to an acceleration detector, and particularly relates to an acceleration detector suitable for controlling a car or the like.
Heretofore, as an acceleration detector, there have been proposed a piezoelectric acceleration detector using a piezoelectric effect of a piezoelectric material, a distortional-gauge acceleration detector using a piezo-resistance effect, a servo acceleration detector having a force feedback mechanism, a magnetic acceleration detector using a differential transformer, an optical acceleration detector using a photointerrupter, a capacitance-operated acceleration detector using capacitance between electrodes formed by evaporation coating, and the like.
Of those acceleration detectors, the capacitance-operated acceleration detector is considered to be most promising for the future.
The capacitance-operated acceleration detector is described in the publication "Transducers '87, Digest of Technical Papers", pages 395-398, 1987.
According to this publication, a displaceable mass is attached to a free end of a cantilever so that the mass displaces upon application of acceleration.
Further, electrodes are provided at the mass portion and a fixed portion opposite to the mass portion to thereby form a capacitance. The displacement of the mass is detected on the basis of the change of capacitance between the mass portion and the fixed portion and the result of detection is fed back to thereby forcedly keep the mass portion at a neutral point by electrostatic force.
Accordingly, the value of acceleration is detected on the basis of the feedback quantity at this time.
In the aforementioned, capacitance-operated acceleration detector, the sensitivity of detection is high but the range of measurement is narrow. To widen the range of measurement, a high driving voltage is required.
However, a first problem arises in that a boosting circuit or the like is required in the case where the value of source voltage is restricted, for example, in a car or the like.
Further, a second problem in, such an acceleration sensor arises from the fact that the gap between the electrode and the mass portion is of the order of microns so that, the gap may be short-circuited in the case where foreign matter invades into the gap.
In this case, an indirect method of testing an electric output has been used for examination of a possible short circuit. There arises a manufacturing problem in that a large time is required for such operation.
This is because, in the currently proposed acceleration sensor, the electrodes are formed of aluminum by evaporating a coating or sputtering, and therefore the electrodes are opaque so that checking cannot be performed from the outside.